For the efficient (and, in some cases, safe) operation of many pieces of large apparatus, it is desirable to know when, at some point in the apparatus, a particular parameter (e.g. temperature, concentration of a particular chemical, pressure) exceeds a particular level. It is also desirable to know where in the apparatus the fault condition (i.e. the excessive value of the parameter) exists. Examples include: excessive temperature at some point along a high voltage electrical cable or in a dryer, e.g. for argricultural products; loss of pressure in a pressurized system, e.g. a telephone cable; and leakage of chemicals from pipe lines or tanks. The use of a plurality of individual point sensors for this purpose is so expensive and inconvenient that it is seldom employed.
More specifically, the sensor of the instant invention comprises (1) an electrical conductor, (2) a dielectric, (3) a solid conductive polymeric composition (CPC), and, optionally, (4) a protective layer all in a geometric relationship such that the dielectric separates the electrical conductor from the CPC and the protective layer, if present, separates the CPC from the environment. This geometric arrangement forms a distributed electrical capacitor with approximately constant capacitance per unit length or per unit area. Under normal conditions, the resistance of the CPC is low enough to allow the capacitor to be charged and its full capacitance to be measured. The CPC is designed to increase its resistance under specified fault conditions such as high temperature, mechanical distortion, fluid saturation, etc. The high resistance at the fault location prohibits the capacitor from being charged or discharged at, or beyond, that location. The result is that a decreased capacitance is measured, indicating a fault condition.
In the case of a one dimensional sensor (cable), a fault at any position along the cable length prohibits the cable beyond the fault from being charged or discharged and the ratio of the measured capacitance to the full cable (unfaulted) capacitance stands in the same ratio as the distance to the fault to the full cable length.
In the case of a two dimensional sensor (sheet), a fault in any location will increase the resistance of a certain area of the sheet, thereby reducing the measured capacitance. The ratio of the measured capacitance to the full sheet (unfaulted) capacitance stands in the same ratio as the fault area to the full sheet area. Such a two dimensional sensor could be placed, for example, around a tank where a one dimensional sensor would provide inadequate coverage.
The instant invention is a significant improvement over the prior art in long line or area fault sending and locating for three basic reasons. First, this invention utilizes a solid CPC which is necessary for the construction of practical sensors possessing flexibility, ruggedness, durability and splicability.
Second, the use of a CPC as the sensor makes possible the construction of a standard sensor and electronics package useful for a wide range of monitoring functions. This is possible by selecting different CPC's and/or different protective layer materials: the internal conductor, dielectric, and electronics may remain the same. This allows for manufacturing standardization and cost savings. For example, an over-temperature monitor will require a CPC whose resistance rises sharply with temperature at a critical temperature level. Protective layer material is not critical in this application. A leak detector could use exactly the same conductor, dielectric, and electronics package as the over-temperature monitor, but require a CPC whose resistance rises sharply at a given concentration of specified fluids. The protective layer could be critical in the leak application if sensitivity only to a specific fluid id desired.
Third and most important, the instant invention provides a massive resistance increase for small changes of the parameter of interest. This resistance change combined with a capacitance measuring technique makes it possible to identify faults in an unambiguous way. For example, a temperature sensitive CPC can exhibit four or more orders of magnitude resistance increase over a small temperature range (5.degree.-10.degree. C.) and eight to ten orders are possible. Hydrocarbon sensitive or moisture sensitive CPC's also exhibit substantial resistance increases with minor hydrocarbon concentrations and over ten orders of magnitude with saturation. These large resistance changes are necessary to electrically isolate the fault thereby preventing the fault region from charging or discharging significantly during the measurement time cycle. For a one dimensional (cable) sensor, this ensures that the portion of cable beyond the fault does not contribute to the capacitance between the control end and the fault, thus making precise fault location possible. For a two dimensional (area) sensor, this ensures that the faulted area does not contribute to the measured capacitance, thus making precise measurement of the fault area possible.